page_title: Swash
section_title: Beach cusps
hierarchical_section_title: Swash / Swash morphology / Beach cusps
caption_attribution_description: English: Cusp morphology showing divergence of the uprush at cusp horns and convergence of the backwash in cusp embayments.
context_page_description: Swash, or forewash in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach materials up and down the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes depending on the type of beach. Greater swash generally occurs on flatter beaches. The swash motion plays the primary role in the formation of morphological features and their changes in the swash zone. The swash action also plays an important role as one of the instantaneous processes in wider coastal morphodynamics.
  There are two approaches that describe swash motions: swash resulting from the collapse of high-frequency bores on the beachface; and swash characterised by standing, low-frequency motions. Which type of swash motion prevails is dependent on the wave conditions and the beach morphology and this can be predicted by calculating the surf similarity parameter εb:
  Where Hb is the breaker height, g is gravity, T is the incident-wave period and tan β is the beach gradient. Values εb>20 indicate dissipative conditions where swash is characterised by standing long-wave motion.
context_section_description: The beach cusp is a crescent-shaped accumulation of sand or gravel surrounding a semicircular depression on a beach. They are formed by swash action and more common on gravel beaches than sand. The spacing of the cusps is related to the horizontal extent of the swash motion and can range from 10 cm to 50 m. Coarser sediments are found on the steep-gradient, seaward pointing ‘cusp horns’ (Figure 3). Currently there are two theories that provide an adequate explanation for the formation of the rhythmic beach cusps: standing edge waves and self-organization.
caption: Figure 3. Beach cusp morphology. Uprush diverges at the cusp horns and backwash converges in the cusp embayments. (Modified from Masselink & Hughes 2003)
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